Modern mass transit door systems typically employ a fluid or pneumatically operated obstruction sensing system which is secured to the leading edge of an automatic door. The most popular system is the pneumatic operating sensing arrangement. Examples of two types of these systems are disclosed in U.S. Pat. Nos. 4,133,365 and 3,303,303, incorporated by reference herein.
Typical arrangements employ a soft hollow extruded rubber or other elastomeric edge that is air tight except for a sensing tube extending from a hollow interior of the edge to a remotely located pressure wave switch. The pressure wave switch includes a diaphragm mounted in a pressure chamber. Switch contacts are mounted on the diaphragm and the pressure chamber. The chamber in the sensing edge is a large relative to the pressure chamber in the pressure wave switch such that the switch is highly sensitive to differential volume variations or changes in the shape of the leading edge. For example, if an arm of a passenger were engaged by the sensing edge, the change in configuration of the chamber in the edge would send a pressure pulse through the sensing tube that is sensed in the pressure chamber in the switch. This pressure pulse causes the diaphragm to flex and the switch contacts to engage, actuating the switch to recycle the system and open the door.
Although existing pressure wave switches generally provide adequate operation, substantial problems arise in the reliability of the electrical contact areas of the switch. Since engagement of the sensitive edges of the doors with a vehicle passenger is utilized to actuate the pressure wave switch and prevent further closure of the door, the importance of reliability of the switch contacts cannot be over estimated.
Switch contact problems experienced by pressure wave switches most commonly in use today arise from the adjusting feature of the adjustable contact included in the switch. The adjustable contact includes threads allowing the contact to be threaded into a tapped terminal link. Since thread engagement between the adjustable contact and terminal link carries the signal current indicating engagement of the vehicle door sensitive edge with an obstruction, it is necessary that the mating surfaces of these threads be maintained in intimate contact.
It has been determined that normal thread tolerances and adjustment requirements of the adjustable contact, often after a period of time, result in the development of high resistance resulting in malfunction of the door edge obstruction sensing function. High resistance can also occur in existing switches since there is no thread bias on the adjustable contact. The adjustment requirements and lack of thread bias result in discontinuity in the threads, and due to low mechanical pressure on the threads, oxidation occurs producing further discontinuity. Reliability of mass transit door systems can be beneficially increased by maintaining mechanical engagement of the terminal link threads with the threads of the adjustable contact.
Additional problems in existing pressure wave switches can occur in a fixed orifice located in a relief vent in the switch housing communicating the pressure chamber with atmosphere. The orifice controls flow through the relief vent allowing the pressure pulse introduced into the pressure chamber upon contact of the elastomeric door edge with an obstruction to flex the diaphragm and actuate the switch. The pressure in the chamber is then vented allowing the diaphragm to reset. The orifice must remain open and clear during the service life of the switch since a clogged orifice will prevent resetting of the diaphragm and malfunctioning of the switch. Typical prior art fixed orifices are not self-cleaning and vapors passing through the fixed orifice tend to condense and particles become lodged, clogging the orifice. Once the orifice is clogged, the diaphragm will not reset and the switch fails to perform its intended function. It is desirable to provide a self-cleaning, fixed orifice thereby extending the service life and increasing the reliability of the switch.